A circuit dimension required for a semiconductor element becomes finer and finer with high integration and large capacity of a Large Scale Integration (LSI).
Using an original image pattern (referring to a mask or a reticle, hereinafter collectively referred to as a mask) in which a circuit pattern is formed, a circuit is formed by exposing and transferring a pattern onto a wafer with a reduced-projection exposure apparatus that is called a stepper or a scanner, thereby reproducing the semiconductor element.
It is necessary to improve yield in the costly production of the LSI. Furthermore, the state-of-the-art device is brought into a stage where the formation of the pattern having a line width of ten-odd nanometers is required. At this point, a shape defect of a mask pattern is cited as a large factor in contributing to degradation of the yield. Specifically, examples of the shape defect include irregularity (edge roughness) at a pattern edge, a line width abnormality of the pattern, and a gap abnormality with the adjacent pattern due to a position deviation of the pattern.
The shape defect of the mask pattern becomes finer with the finer dimension of an LSI pattern formed on a semiconductor wafer. Because fluctuations of process conditions are absorbed by enhancing dimension accuracy of the mask, it is necessary to defect an extremely small defect of the pattern in a mask inspection. Therefore, high accuracy is required for an apparatus that inspects the pattern of a transfer mask used in the LSI production. Japanese Patent No. 4236825 discloses an inspection apparatus that can detect fine defects on the mask.
Nanoimprint Lithography (NIL) attracts attention as a technology for forming the fine pattern. In the technology, a mold (a die) having a nanoscale fine structure is pressed onto a resist on the wafer with a pressure to form the fine pattern in the resist.
In a nanoimprint technology, in order to improve productivity, a plurality of replicated patterns (hereinafter referred to as daughter patterns) are prepared using a master pattern that constitutes an original plate, and the daughter patterns are used while mounted on different nanoimprint apparatuses. It is necessary that the daughter pattern be produced so as to correspond collectively to the master pattern, and it is necessary to inspect both the master pattern and the daughter pattern with high accuracy in an inspection process.
At this point, generally the mask is formed with the dimension four times the circuit dimension. After the pattern is reduced and exposed on the resist on the wafer with the reduced-projection exposure apparatus using the mask, development is performed to form the semiconductor circuit pattern. However, in the nanoimprint lithography, the master pattern and the daughter pattern are formed with the same dimension as the circuit dimension. For this reason, the shape defects in the master pattern and the daughter pattern have a large influence on the pattern transferred onto the wafer compared with the pattern of the mask. Accordingly, compared with the pattern of the mask, higher accuracy is required for the inspections of the master pattern and the daughter pattern.
In the inspection apparatus, the sample is irradiated with light emitted from a light source through an optical system. The sample is placed on a table, and the sample is scanned with the light by moving the table. An image of the light transmitted through or reflected from the sample is formed on an image sensor via a lens. The defect of the sample is inspected based on optical image data obtained by the image sensor.
However, in the master pattern and the daughter pattern, the pattern dimension is smaller than the resolution of the optical system in the inspection apparatus. Therefore, it is difficult to accurately detect the focal position. For example, in a focal position detection method in which an optical level method is adopted, the sample is irradiated with the light from the light source through an objective lens, and the image of the image of the light reflected from the sample is formed on a position sensor. Therefore, a displacement amount from a focal position is obtained such that contrast reaches the maximum, and control is performed so as to properly obtain a distance between the sample and the objective lens. However, when the line width of the pattern on the sample is less than or equal to a wavelength, the image of diffracted light generated in the pattern is formed on the position sensor, but the displacement amount cannot accurately be obtained.
It is conceivable that the focal position is measured by providing reference planes on four corners in an inspection area of the sample, and that the focal position is adjusted in the inspection area based on the reference planes. However, when pressure or temperature changes in the inspection process, the focal position of the light with which the sample is irradiated fluctuates height data in the reference plane. For example, because a refractive index of air changes when the pressure changes, an imaging plane of an object, namely, the focal position changes. Accordingly, it is difficult for the focal position to always be accurately adjusted by this method.
The present invention has been devised to solve the problems described above, and an object of the present invention is to provide a focal position detection apparatus and a focal position detection method, which can detect the focal position of the sample having a repetitive pattern in which the period is less than or equal to the resolution of the optical system.
Another object of the present invention is to provide an inspection apparatus and an inspection method, which can detect the defect in the repetitive pattern in which the period is less than or equal to the resolution of the optical system.
Other challenges and advantages of the present invention are apparent from the following description.